DDS waveform generation has become the standard method for generation of complex modulated waveforms. DDSs are a primary building block for modern waveform generators. Typically, the DDS generates the modulated waveform and a frequency translation is performed to up-convert the signal to a frequency within the operating band. Up-conversion may be performed through frequency multiplication or frequency mixing. Frequency multiplication is a convenient method for increasing frequency and expanding bandwidth. However, frequency multiplication suffers a 20logN degradation to both phase noise and spurious in the output. Frequency mixing is typically used for narrow bandwidth applications. In wideband applications, frequency mixing produces spurious mixing products that become a significant limitation for spurious performance for wide bandwidths.
When considering performance of DDSs, emission of spurious signals in the output is an important consideration as low spectral purity is a primary performance metric. Spurious signals may result from either non-linearities in the DDS itself or spurious energy signals may be created in other devices that perform the frequency translation (e.g. a frequency mixer). For example, where frequency mixing is performed, mixing products create spurious signals due to the multiplication of local oscillator (LO) and intermediate frequency (IF) (e.g. clock frequency of the DAC) harmonics. These spurious products occur at frequencies that are integer multiples of the LO frequency and the IF frequency according to:mLO+/−nIFwhere m and n represent multiples of the LO frequency and IF frequency, respectively.
The implications of these spurious signals in system development require the development of a frequency plan which accounts for anticipated spurious signals. Selection of LO/IF clock frequencies, filters, etc. are directed to minimizing in-band spurious while meeting operation band requirements. For narrow band applications, available components are adequate to meet low spurious performance. However, in wideband applications, currently available components are inadequate to provide needed spurious performance. Furthermore, the DDS spurious may be directly in band, thereby presenting additional problems.
Alternative methods and devices which address spurious energy signals in waveform generators are desired.